Molecular-level carbon traits of fine roots: unveiling adaptation and decomposition under flooded condition

Abstract. Fine roots constitute a fundamental source of litter decomposition and humus formation in terrestrial ecosystems. However, molecular-level traits of carbonaceous organics in fine roots grown in different media, such as soil and water, remain largely unexplored, which limits our understanding of root adaptation and decomposition under changing environments. Here, we used a sequential extraction method to obtain dichloromethane-and-methanol-extractable (F_DcMe), base-hydrolyzable (F_KOHhy), and CuO-oxidizable (F_CuOox) fractions from fine roots of Dysoxylum binectariferum grown in soil and water and characterized them using targeted gas chromatography-mass spectrometry and non-targeted Fourier transform ion cyclotron resonance mass spectrometry. Also, decomposition experiments were conducted on soil- and water-grown roots under aerobic and anoxic conditions. Results showed a consistent increase in unsaturation degree and aromaticity of the analytes from F_DcMe to F_CuOox fractions. Both analyses were sufficiently sensitive to show that compared to soil-grown roots, the water-grown ones developed more polyphenolics with a high unsaturation degree and aromaticity and had more non-structural compositions. Furthermore, although flooding provided an anoxic condition that slowed down root decomposition, the adaptive strategy of developing more non-structural labile components in water-grown roots accelerated root decomposition, thereby counteracting the effects of anoxia. Our results highlight that the complementary targeted and non-targeted analyses of sequentially extracted fractions can provide the supplementary molecular-level carbon traits of fine roots. It advances our understanding of biogeochemical processes in response to global environmental change.